A dynamic range is one of important factors to determine the quality of an image sensor. In general, the dynamic range refers to the maximum range for processing signals without distorting input signals. In the case of the image sensor, images having the superior quality can be obtained as the dynamic range becomes widened regardless of the brightness variation.
However, according to the color image sensor of the related art, the dynamic range is so narrow that the original color of the image may not be expressed well when one of red, green and blue colors is saturated. In order to solve the problem caused by the narrow dynamic range, a WDR (wide dynamic range) pixel has been suggested.
For instance, there has been suggested a method of realizing the WDR operation by adjusting irradiation time of the light in the image sensor of the related art.
In addition, there has been suggested a method of providing an additional capacitor to change FD (floating diffusion) capacity, in which a pixel structure includes a transistor to adjust the additional capacitor so that overflow charges, which are generated from a PD (photodiode) under the high intensity of illumination as light intensity is increased, can be stored in the additional capacitor.
Further, there has been suggested a method of providing a WDR pixel, in which two PDs are installed in one pixel such that charges generated from the two PDs are combined with each other.
However, according to the above methods, the sensitivity is constant regardless of the variation of light intensity (that is, high intensity of illumination and low intensity of illumination), so that the image may be darkened under the low intensity of illumination. In addition, while the pixel is being operated, the timing adjustment for the pixel operation under the high intensity of illumination may be limited.
Further, in the case of the method for improving the sensitivity according to the light intensity by using the additional capacitor and the transistor and the method for driving two PDs installed in one pixel, the fill factor in the pixel may be reduced.